1. Field of the Invention
The present invention relates to a process for producing a modified fibrous material useful for reinforcing rubber articles. More particularly, the present invention relates to a process for producing a modified fibrous material comprising aromatic polyamide fibers and/or polyester fibers having an enhanced bonding property to rubber and an excellent heat resistance, and thus useful as a reinforcing material for rubber articles. Still more particularly, the present invention relates to a process for producing a modified fibrous material comprising aromatic polyamide fibers and/or polyester fibers, and useful for producing a fibrous material-reinforced rubber composite article having an enhanced rubber coverage on the fibrous material when the fibrous material is peeled from the composite article, a satisfactory softness, and an excellent resistance to fatigue.
2. Description of the Related Art
It is known that aromatic polyamide fibers exhibit a higher tensile strength, modulus, heat resistance, water resistance, and dimensional stability at an elevated temperature or in water than other ordinary synthetic polymeric fibers and, therefore, are useful as reinforcing materials for rubber articles, for example, tires, belts, and hoses.
Also, it is known that polyester fibers, for example, polyethylene terephthalate fibers, exhibit a high tensile strength, Young's modulus, low elongation and creep, and an excellent resistance to fatigue and, therefore, are useful as reinforcing materials for rubber articles. Further, it is known that the naked surfaces of the aromatic polyamide fibers and the polyester fibers exhibit a poorer bonding property to rubber than other ordinary synthetic fibers, for example, nylon 6 fibers and nylon 66 fibers, which are usually utilized as reinforcing fibers for rubber articles. This is because the affinity of hydrogen bonds in the ester structure in the polyester molecules and in aromatic amide structures in the aromatic polyamide molecules to the rubber, is poorer than that of other ordinary polymer molecules. Accordingly, it is necessary to enhance the bonding property of the aromatic polyamide and polyester fibers to rubber by a specific surface treatment.
In a conventional surface treatment, the surfaces of the aromatic polyamide and polyester fibers are coated with an adhesive material, for example, an epoxy compound, isocyanate compound or a mixture (RFL) of a resorcinol-formaldehyde resin and a rubber latex. However, the conventional adhesive materials are not satisfactory for use with the aromatic polyamide and polyester fiber materials.
There have been various attempts to enhance the bonding property of the aromatic polyamide and polyester fiber materials to rubber. For example, before the surface treatment with the resorcinol-formaldehyde-rubber latex (RFL), the fibrous material is pretreated with an aqueous solution or dispersion of an epoxy compound, or with a polyamide or polyamide derivative. This attempt does not always satisfactorily enhance the bonding property of the fibrous material. Furthermore, it is known that the surface-treated fibrous material by the conventional treatment exhibits an undesirably increased stiffness, a degraded molding property, and a decreased resistance to fatigue.
The Journal of Applied Polymer Science, Vol. 18, pages 1557 to 1574 (1974) discloses a process for improving the bonding property of a polyester fiber material to rubber, comprising the steps of applying a low temperature plasma treatment to the polyester fiber material under a reduced pressure to activate the fiber surfaces thereof, and impregnating the plasma-treated polyester fiber material with a resorcinol-formaldehyde-rubber latex (RFL).
Japanese Unexamined Patent Publication No. 61-19881 discloses a process for enhancing the bonding property of a polyester or aromatic polyamide fiber material to rubber by a low temperature plasma treatment and a resorcinol-formaldehyde-rubber latex-treatment.
Japanese Unexamined Patent Publication No. 60-250036 discloses a process for bonding an aromatic polyamide fiber material to a rubber material by applying a low temperature plasma treatment to the fiber material and by bonding the plasma-treated fiber material to the rubber material with an adhesive composition comprising a resorcinol-formaldehyde precondensation product having a high molecular weight and a rubber latex.
The above-mentioned attempts satisfactorily enhance the initial bonding property of the fiber material to the rubber material but do not give a satisfactory heat resistance of the resultant reinforced rubber article in practical use.